Solder is generally used for bonding metal materials and is composed of a metal alloy material having a melting temperature range (a range from the solidus temperature to the liquidus temperature) of 450° C. or less. A Sn-37Pb eutectic solder having a melting point of 183° C. has heretofore been generally used as a solder material used in reflow heat treatment. Further, a Sn-90Pb high temperature solder having a solidus temperature of 270° C. and a liquidus temperature of 305° C. has been extensively used as a high temperature solder used in the interior of electronic parts and the like where high heat resistance is required.
However, in recent years, the harmfulness of lead is becoming a problem as described in the EU's environmental regulations (WEEE and RoHS directives), and the percentage of a lead-free solder in the total solder is rapidly increasing from the viewpoint of preventing environmental pollution. At present, in such a situation, there is proposed a lead-free solder (refer to Patent Documents 1 and 2) comprising Sn-3.0Ag-0.5Cu having a melting point of about 220° C. as an alternative to the Sn-37Pb eutectic solder, and reflow heat treatment in the range of about 240 to 260° C. is coming into common use. In general, conditions of reflow heat treatment are set in a temperature range higher by 10 to 50° C. than the melting point of solder metal alloy.
On the other hand, an Au-20Sn eutectic metal alloy (melting point: 280° C.), a Sb—Sn based metal alloy (refer to Patent Document 3) and the like are proposed as a high temperature solder material. However, the Au—Sn based metal alloy is limited in use because it is a material which is mechanically hard and brittle and thereby has poor stress relaxation characteristics, and it is also an extremely expensive material. Further, the Sb—Sn based metal alloy is pointed out that it has a harmfulness of Antimony. Thus, there is no strong alternative material for a high temperature solder material. As a result, in the above-mentioned RoHS derivatives to be executed from July, 2006, a high temperature solder containing 85% or more of lead is granted a waiver until an alternative material is established.
The inventors have proposed a lead-free connection material simultaneously satisfying reliability with respect to heat resistance (even when the connection material is repeatedly subjected to heat treatment for mounting electronic parts and the like, the connection material is capable of preventing displacement of the electronic part which has already been mounted on the substrate), connection stability (capable of maintaining the stand-off between the electronic part and the substrate), and capability of attaching an electronic part or the like to a substrate by heat treatment at about 250° C. or less (refer to Patent Document 4). The connection material was composed of metal alloy particles having a two-layered structure, wherein the surface of the metal alloy particles had a composition different from the interior thereof, and the connection material had a feature that when it is used for connection, only the surface melts while the interior does not melt to thereby develop connection stability, and after connection the lowest melting point increases to thereby develop reliability with respect to heat resistance. As a specific preferred embodiment, there have been proposed metal alloy particles in which the metal alloy has a metastable phase composed of copper, tin, silver, bismuth, and indium and is coated with tin on the surface thereof by displacement plating.
Further, the inventors have proposed, as a lead-free connection material having the same object, a lead-free connection material comprising a composition of first metal alloy particles and second metal alloy particles, wherein the first metal alloy particles are obtained by subjecting the surface of the above-mentioned metal alloy having a metastable phase to displacement plating of tin and the second particles contain tin and indium and have a lowest melting point of from 50 to 150° C. (refer to Patent Documents 5, 6 and 7).
The solder material using the metal alloy particles as mentioned above can not only be used as a lead-free material but also satisfy a requirement as a high temperature solder material to a certain extent because the lowest melting point increases after the solder material is melted once and cured. However, in order to produce the metal alloy particles, a process to form a low melting point layer, for example, a tin displacement plating layer on the surface thereof is required, and the process had a problem of low productivity. In addition, since the displacement plating process proceeds by replacing copper on the surface with tin, it was technically difficult to increase the thickness of the displacement plating layer for further improving the connection strength.
Further, when a connection material comprising a composition containing metal alloy particles having a lowest melting point of from 50 to 150° C. such as the above-mentioned second metal alloy particles is heated to 240 to 260° C. at the mounting, the growth of the crystal grains of the metal alloy structure may proceed to thereby reduce the bonding strength due to the internal stress thereof. Therefore, it is not preferred that the difference between the lowest melting point and the mounting temperature is large.
[Patent Document 1]: Unexamined Japanese Patent Application Laid-Open Specification No. Hei 5-050286
[Patent Document 2]: Unexamined Japanese Patent Application Laid-Open Specification No. Hei 5-228685
[Patent Document 3]: Unexamined Japanese Patent Application Laid-Open Specification No. Hei 11-151591
[Patent Document 4]: WO 2002/028574
[Patent Document 5]: Unexamined Japanese Patent Application Laid-Open Specification No. 2004-223559
[Patent Document 6]: Unexamined Japanese Patent Application Laid-Open Specification No. 2004-363052
[Patent Document 7]: Unexamined Japanese Patent Application Laid-Open Specification No. 2005-5054